The present invention relates to picture signal processing method.
A display apparatus is provided with a displaying screen having a predetermined aspect ratio. On the other hand, there has been in practical use a system for displaying a letter-box type picture in the display apparatus, which system is capable of displaying a picture having another aspect ratio different from the above predetermined aspect ratio.
For example, according to NTSC (National Television system committee) standard system, there has been in use a television receiver which has a display screen having an aspect ratio of 4:3. In order that such a television receiver may be used to display a wide Picture having an aspect ratio of 16:9 which is just the same as that of a motion picture, a kind of letter-box type picture signal has been used in the television broadcast.
When using the letter-box type picture signal, as shown in FIG. 8, the display screen of a television receiver having an aspect ratio of 4:3 is allowed to display a picture which is comprised of i) a main picture portion A having an aspect ratio of 4:(9/4) corresponding to an aspect ratio of 16:9, ii) two non-picture portions B1, B2 each having an aspect ratio of 4:(3/8). In detail, a wide picture may be displayed on the main picture portion A, while the non-picture portions B1 and B2 are made black without showing any picture. In this manner, although the whole picture will be reduced in its size, it is sure to display a wide picture having an aspect ratio of 16:9 without missing any portions of the picture.
Further, according to NTSC standard system, one frame picture information is divided into two fields so as to be transmitted. On the television receiver side, the picture information of the two fields is scanned in an interlace scanning treatment so as to reproduce said one frame picture information. Such kind of principle is also suitable for use in transmitting a letter-box type picture. Namely, one frame picture information is divided into two fields so as to be transmitted. On the television receiver side, the picture information of the two fields is scanned in an interlaced scanning treatment so as to reproduce said one frame picture information, in a manner as shown in FIG. 8.
However, when the above interlace scanning is performed to reproduce picture information, since the scanning is carried out on every other line, a reproduced picture will become rough or some line flickerring phenomenon will occur, resulting in a low picture quality. In order to solve this problem, it has been suggested that on the television receiver side, an interpolation treatment be performed so that the interlace scanning treatment may be changed to a non-interlace scanning treatment (progressive scanning). One example of such non-interlace scanning treatment is called motion adaptive interpolation method.
According to the motion adaptive interpolation method, with respect to a still picture, an inter-field interpolation treatment is performed so that the picture information involved in two fields may be used to produce interpolation scanning signal. On the other hand, with respect to a moving picture, an in-field interpolation treatment is performed, so that scanning line information of mutually adjacent scanning lines may be used to produce interpolation scanning signal. The above different kinds of interpolation treatments may be combined together and be properly changed over between each other in accordance with a moving amount of a picture, thereby ensuring a good picture quality for a reproduced picture.
However, when the motion adaptive interpolation method is used to process the letter-box type picture signal, moving picture and still picture will be undesirably mixed together in boundary portions L1, L2 between the main picture portion A and the non-picture portions B1, B2. As a result, although it is possible to perform the interpolation treatment on moving picture portions in the boundary portions L1, L2, it is impossible to perform the interpolation treatment on still picture portions. Consequently, the boundary portions L1, L2 will contain clearly visible portions on which the interpolation treatment has been performed and non-visible portions on which the interpolation treatment has not been performed, hence resulting in a low picture quality.